An insider at the bash, held at the Mondrian Hotel’s swanky Skybar in LA, caught Crystal, 24, kicking back with her dad, some friends AND the sexy rocker — whom she once confided to have a crush on. Hmmmm…..

Kara DioGuardi wasn’t sniffing around Casey, strangely, but paying tribute to Simon Cowell on his last American Idol evening. Host Ryan Seacrest, former judge Paula Abdul and current judge Randy Jackson all abandoned the groups they came with to hang with the Brit. Even Ryan’s parents joined in the festivities, mingling with a crowd that included new BFFs Aaron Kelly and season seven runner-up David Archuleta.

Who else came to celebrate Lee’s big win? Brooke White, Michael Johns, Jason Castro and Melinda Doolittle came rounded out the Idol experience.